1. Field of the Invention
The present invention relates to a magnetic resonance apparatus in which a magnetic metal part is accommodated in an accommodation section for correction of uniformity in a main magnetic field.
2. Description of the Related Art
A magnetic resonance apparatus is provided with a magnet (a permanent magnet or an electromagnet) for generating a main magnetic field and a gradient magnetic field system (a gradient coil) for generating a gradient magnetic field. The main magnetic field is a static magnetic field and should desirably have high uniformity. In order to maintain the uniformity of the main magnetic field, shimming is performed. Shimming is roughly classified into passive shimming and active shimming. In the passive shimming, a magnetic metal piece (iron piece or the like) called a shim is arranged in the vicinity of the magnet, thereby adjusting magnetic field distribution of the main magnetic field. More specifically, arrangement of a plurality of magnetic metal pieces is contrived, whereby uniformity of the main magnetic field is maintained. In the active shimming, by adjusting a current to be caused to flow through a coil (shim coil), a correction magnetic field for uniformizing the main magnetic field is generated.
Then, in the passive shimming, it is known that non-uniformity of the main magnetic field is caused by variation of the temperature of the magnetic metal piece. It is also known that an offset is caused in the main magnetic field by the variation of the temperature of the magnetic metal piece. As a cause of the temperature variation, heat generation from a gradient coil and heat generation caused in a magnetic metal by an eddy current induced by generation a gradient magnetic field are mainly considered. When the temperature of the magnetic metal is changed by such heat generation, the magnetic susceptibility of the magnetic metal is changed and, as a result of this, the intensity of the main magnetic field is changed locally or entirely.
A technique is known in which a correction magnetic field for correcting non-uniformity or the like of the main magnetic field is included in a gradient magnetic field (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2-206436). In this technique, a temperature of a magnet is detected, and a correction amount is determined in accordance with the temperature. Further, an offset of a current value corresponding to the correction amount is added to a current for generating a gradient magnetic field which is originally required, and the resultant current is supplied to a gradient coil.
In the above-mentioned prior art technique, the main magnetic field is corrected by the gradient magnetic field, and hence the correction amount and the correction resolution have their limits, and there has been no guarantee that the uniformity and intensity of the main magnetic field could have been kept constant.